Stable signals with precisely specified constant frequencies are required in many applications including microprocessor technology, video applications and telecommunications. A very precise, constant output reference frequency is particularly essential in the context of voltage-controlled oscillators (VCOs) and especially voltage-controlled crystal oscillators (VCXOs), which are frequently used for example in mobile communications as part of a phase locked loop (PLL). Such PLLs supply the carrier frequency, for example, which is required as input signal for mixers.
For example in base stations of digital networks, the reference oscillator constitutes a core component, which has on the one hand to meet extremely stringent conditions with regard to accuracy and stability while on the other hand being capable of manufacture in large numbers and at the lowest possible cost.
To achieve the required accuracies, trimming is often necessary. The alternative option of using very tight-tolerance components makes the oscillator circuit very expensive. Mechanical trimming can only be automated to a certain degree, and may result in a reduction in the quality of the oscillating circuit.
In addition to mechanical trimming by grinding of resonators, conductor trace elements or the like or bending of air-core coils to adjust the parameters of an oscillator circuit, it is also known to machine resonators by means of high-energy radiation, such as laser radiation for example, to adjust the oscillator parameters. Trimming of a capacitor constituting part of an oscillator circuit is disclosed in U.S. Pat. No. 4,095,199, for example.
A method of trimming capacitors is disclosed in U.S. Pat. No. 5,264,983. The method includes providing a monolithic capacitor with a top electrode, a part of which may be removed by high-energy radiation, such as for example laser radiation. The particular advantage of this procedure is that the laser device may be actuated fully automatically as a function of the capacitance values achieved. In addition, markedly more accurate trimming may be achieved than with a conventional mechanical method. This is clear for example from the Johanson Technology website,
http://www.johansontechnoloy.com/products/lzc/index.htm
where a trimmable capacitor is used as a resonant element in a resonant circuit.
However, known solutions for automatic trimming of the resonant characteristics of an oscillator circuit have the problem that the use of trimmable components in the actual resonant circuit generally results in a marked reduction in the quality of the resonant circuit.